The present invention relates to a diagnostic method for contactors. It relates in particular to a diagnostics method for diagnosing the operation of contactors used to determine the position of a motor vehicle brake pedal.
Publication EP-0 428 338 describes a method of controlling a motor vehicle cruise control. During implementation of this method, a reduction in the speed of the vehicle below a predetermined threshold is detected. If this reduction takes place within a predetermined space of time without the brake contactors indicating pressure by the driver on the brake pedal, the contactors are considered to be defective.
A problem with this type of diagnostics lies in the fact that detection of the fault of the brake contactors is performed only under set conditions. Thus, for example, the vehicle speed needs to have dropped by 30 km/h in under two seconds in order for a fault to be detected.
Now, brake contactors may malfunction at any time whatsoever in the life of the vehicle and it is desirable to know their operating state in circumstances other than those defined in publication EP-0 428 338.
FIG. 1 is a schematic diagram of a brake pedal 1 equipped with two contactors A and B for determining the position of the brake pedal 1. The brake pedal 1 is also equipped with a return spring 3 which keeps it in a rest position when not actuated by a user.
As may be seen in FIG. 2, the information regarding the position of the brake pedal is transmitted to an electronic control unit (ECU) which processes it in order to determine the position of the pedal. Coherence between these signals also allows the ECU to determine the operating state of the contactors A and B.
One example of the signals supplied by the contactors A and B to the electronic control unit is depicted in FIG. 3. As a function of the logic signals 42 and 44 emitted by the contactors A and B, the electronic control unit interprets the state of the brake pedal. When the contactor A is supplying a high logic state and the contactor B is supplying a low logic state (zone 1), the ECU interprets that the brake pedal is in a state known as the released state. When the contactor A is supplying a low logic state and contactor B is supplying a high logic state (zone 3), the ECU interprets that the brake pedal is in a state known as the depressed state.
At the present time, the contactor diagnostics are based on the coherence of the signals they supply to the ECU. Thus, when the signals (42, 44) coming from the two contactors are supplying a low logic state (zone 2), the ECU interprets these signals as meaning that the brake pedal 1 is both depressed and released. This situation arises during a phase known as a transient phase, when a user depresses the brake pedal just as the contactors change position because they do not switch over exactly at the same moment. This then is a state known as an incoherent state and this incoherence may be interpreted as a fault with one of the contactors when it lasts for too long. However, a driver may also enter and remain in this zone 2, known as the incoherent zone, when he rests his foot lightly on the brake pedal. This may also arise when the vehicle is stationary (for example at a red light) or when it is driving along if the driver prepositions his foot over the brake pedal in anticipation of a need to brake. In all these scenarios, there is not necessarily a fault with one of the contactors A and B.
It is therefore important for the ECU to be able to tell the difference between a transient state of the contactors A and B and a true fault because in the latter instance it is necessary to let the repair shop and/or the driver know that a repair is needed.
In order to solve this problem, it has been proposed that the ECU should start a time out when the incoherent zone of FIG. 3 is entered. This makes it possible to take account of the transient state and avoid falsely diagnosing a fault with the contactors.
However, the inventors have found that in some cases, implementing this type of diagnostics still leads to far too many detections of false faults or non-detection of faults.
Furthermore, the diagnostics of the prior art may, under some circumstances, consider the contactors to be operating normally even though they are faulty. For example, in the event of an open circuit on contactor A, the contactors are once again considered to be operating as soon as the driver brakes. This causes a transition from the incoherent zone to the braking zone. However, contactor A is still faulty.